In recent years, there have been proposed and developed various hydraulically-operated vane rotor equipped variable valve timing control (VTC) devices, capable of locking a vane rotor at a predetermined intermediate-phase angular position (simply, an intermediate position) between a maximum phase-advance position and a maximum phase-retard position by means of a lock pin, when an internal combustion engine stops. One such hydraulically-operated vane rotor equipped variable valve timing control device has been disclosed in Japanese Patent Provisional Publication No. 2010-261312 (hereinafter is referred to as “JP2010-261312”). In the valve timing control device disclosed in JP2010-261312, during an engine stopping period, the vane rotor rotates to the intermediate position by virtue of a fluttering motion of the vane rotor, caused by positive and negative alternating torque, acting on a camshaft due to spring forces of valve springs. Immediately when the vane rotor reaches the intermediate position, the vane rotor is locked and held at the intermediate position by engagement of the lock pin with a lock hole.
For instance, assume that the engine stalls under a specific condition where the lock pin is located nearer the phase-retard side than the intermediate position during a low-temperature engine operating condition in which a viscosity of working fluid is high, and thus the engine is cranked for restarting. Under the previously-discussed specific condition, owing to a viscous resistance of working fluid in phase-retard hydraulic chambers and phase-advance hydraulic chambers, there is an increased tendency for a fluttering motion of the vane rotor to reduce. This undesirably lengthens a traveling time of the lock pin reaching its intermediate lock position, thus deteriorating a startability of the engine.
To avoid this, JP2010-261312 teaches the provision of an auxiliary working-fluid discharge (exhaust) passage through which working fluid in each of the hydraulic chambers is released or exhausted to the outside of the VTC device, thereby increasing a fluttering motion of the vane rotor during the cranking and restarting period, and consequently causing a more smooth and rapid movement of the lock pin toward the intermediate lock position.